Soil Temperature and Day Length

Posted May 9, 2018byMark Macdonald

9 May

The changing seasons, and the longer daylight hours in summer are a result of the angle of the Earth’s axis in relation to the sun. It’s easy to forget how these changes can affect the growth of plants, and in particular, vegetables. Soil temperature plays a very important role in the success or failure of the vegetable garden. Beet seeds, for instance, do not require particularly warm soil to germinate, and they will produce nice leafy tops if sown in early spring, but if the soil is too cold at planting time, they may not produce well-developed roots. By contrast, spinach planted when the soil is too warm may “bolt,” or pass to its flowering stage without producing any leaves.

Leafy vegetables like lettuce, spinach, mustards, arugula, sorrel, pac choi, and cress prefer growing in cooler soils, so we focus on them in spring and again in the fall and, with some protection, in winter. For this group of plants, warm soil represents a stress that signifies the end of the growing season. The plants urgently send up flower stalks and devote their remaining energy to setting seed. Because of this, their leaves tend to become tough, bitter, or unpalatable. When you see references to “bolt-resistance” in lettuce, it’s because the variety has been bred to resist this phenomenon longer into warm weather.

Aside from warming soil, other stresses can cause plants to bolt. Drought can trigger it, so consistent irrigation is important. Some plants (dill, for instance) may bolt when they are transplanted – the shock of root disturbance and cooler nighttime temperatures will sometimes produce very short plants with flowers but few leaves. To avoid this, dill growers usually direct sow their seeds in late spring.

Day length also plays an interesting role in the lives of some plants. The phenomenon known as “photoperiodism” dictates which plants bloom at different times of the year. Many plants use a protein in their tissues to detect changes in the number of hours of darkness in each 24-hour period. As nights grow shorter in late spring and early summer, the flowering process is triggered. Plants that fall into this category are called Long-day Plants, and include oats, clover, peas, barley, lettuce, turnips, and many others.

Short-day Plants have the flowering process triggered by the arrival of increasing hours of darkness, so they typically flower after the summer solstice. Plants in this category include tobacco, chrysanthemums, rice, and others. Yet another category of Day-neutral Plants exists (including cucumbers and tomatoes) that bloom regardless of the day length. These plants tend to bloom once they reach a certain stage of maturity, and because of this, they simply don’t bolt.

Onions have the curious distinction of coming in both Long-day and Short-day varieties. Day length triggers bulb formation in onions. Long-day onions need a day length of 14-16 hours in order to trigger the bulbing process in summer, whereas Short-day varieties bulb up when the day length is 12-14 hours. This is an important consideration for growers, and relates to the latitude at which they are grown. Here in BC, and elsewhere in the northern half of North America, we grow Long-day onions. Down in Georgia, where masses of onions are farmed, they grow Short-day types. West Coast Seeds only offers Long-day (and occasionally day-neutral) onion varieties.

Both soil temperature and day length have a big impact on garden plants.